1. Field of the Invention
The present invention relates to a recording method. More specifically, the present invention relates to a method of forming a recorded image in which the fluorescence intensity and recording density of a fluorescent recorded matter obtained by means of fluorescent ink are good and which is excellent in reliability of image formation.
2. Description of Related Art
In recent years, ink which can be used in various applications has been desired. The applications of the ink are not only for the formation of beautiful color images. For example, a technology has been proposed and developed in which information such as characters, numbers, symbols or bar-codes, is recorded on a recording medium using an ink provided with fluorescence properties, the ink is irradiated with UV light of an appropriate wavelength and allowed to emit colored light, thereby providing the recording medium with additional information (such as security information) other than visual information. Among the technological developments, particularly in a system for reading out authentication (anti-counterfeit) information or security information by the use of an apparatus that causes the ink to emit fluorescence and scans the emission intensity thereof, a fluorescent coloring material is excited with light of a standard wavelength (e.g., 254 nm, standard excitation wavelength) used in the system so that fluorescence color development is brought about. The fluorescence color development is used for determination or measurement and applied to a mailing system or the like.
Considering ink in terms of the kinds of coloring materials, when a dye is used as a coloring material, the ink can be easily provided with a standard color tone. In some cases, however, the water resistance of the obtained image is inferior. In contrast, when a pigment is used as a coloring material, a standard color tone may not be obtained while resulting in good water resistance of the obtained image. In view of the above, an ink is proposed containing both a dye and a pigment as an ink capable of providing an image that satisfies both water resistance property and color tone. For example, JP-B-60-45669 (Patent Document 1) discloses a recording liquid that contains a water-soluble red dye (e.g., Acid Red 52) and a red pigment as recording agents, and a polymer dispersant for dispersing these recording agents into a liquid medium.
On the other hand, as a mailing system, fluorescent red is commonly printed in the U.S, and a dye such as Acid Red 52 (AR52), which is described in the above publication, is used as a fluorochrome. U.S. Pat. No. 6,176,908 (Patent Document 2) exemplifies the AR52 as a fluorescent dye and discloses an ink that contains the fluorescent dye, a pigment, and a polymer as a dispersant of the pigment. Furthermore, as described herein, in order to change the formed color tone of the “tint of an image” (i.e., for adjusting the color tone identified by the sense of a human being), the combination of dyes for matching to such a color tone has been known as a matter of design long before the description of Patent Document 2.
Furthermore, the invention disclosed in U.S. Pat. No. 6,176,908 (Patent Document 2) relates to an ink-jet ink in which a pigment is added to a fluorescent dye for improving the water resistance of the ink as in the case of JP-B-60-45669 (Patent Document 1), and there are described a system in which two kinds of fluorescent dyes are combined for the well-known object of visible (visual) tint, and a system in which an additive is added for improving fluorescence intensity (PMU level). As specific examples for improving the fluorescence intensity (PMU level), except for a pigment part (a polymer, tetraethylene glycol, or diethylene glycol), solvents such as water, 2-pyrrolidenone and tetraethylene glycol, and the following fluorescent coloring materials are described. In addition, as the multiple fluorescent coloring materials, examples of a combination of the above AR52 (0.4% by mass, 0.5% by mass to 3.0% by mass) with one of Acid Yellow 7 (AY7), Acid Yellow 73 (AY73) and Direct Yellow 96 (DY96), and the addition of Basic Yellow (BY40) to Basic Violet (RHDB) are described.
Moreover, Japanese Patent Application Laid-Open No. H11-80632 (Patent Document 3) discloses an invisible fluorescent aqueous ink containing three different fluorescent coloring materials (a fluorescent brightening agent, a yellow fluorescent dye based on a coumarin derivative, and a red fluorescent dye based on rhodamine-B or rhodamine-6G), and a postcard print using the ink. In the technical description thereof, each of those three different fluorescent coloring materials generates its peak emission due to UV light, and then other coloring materials are sequentially excited by the emission, finally causing fluorescent emission with a wavelength of 587 nm. In this publication, however, there is no specific description about such an excitation wavelength, and the technical description is performed on the basis of the result that the ink and its recorded image show the same fluorescence characteristics. Generally, water absorbs UV light, hence the fluorescence of a recorded image will be different from that of the ink used. Making a judgment from this fact, therefore, the invention described in the publication lacks credibility from engineering perspective. Furthermore, WO 02/092707 (Patent Document 4) discloses an ink that generates color emission as standard emission while forming a black image using multiple dyes (a red dye, a yellow dye, a blue dye, and a black dye) that emit fluorescence by virtue of excitation with UV light as with the effect of Patent Document 3. This publication describes, as a technical point which is different from that of Patent Document 3, an invention directed primarily to selecting multiple dyes in such a manner that an absorption spectrum in relatively longer wavelengths and an emission spectrum in shorter wavelengths are not overlapped. In this publication as well, the desired fluorescence intensity is not always ensured without fail because the relationship between the multiple fluorescent coloring materials cannot be analyzed sufficiently. On the other hand, Japanese Patent Application Laid-Open No. 2003-113331 (Patent Document 5) discloses the invention for improving the fluorescence characteristics of ink due to the relationship between solvents and fluorescent coloring materials. Patent Document 5 discloses a recording ink that includes two fluorescent coloring materials with the same color (there is also an example in which a coloring material having no fluorescence is added), two different organic solvents (e.g., glycerin and a nonionic surfactant) which have no compatibility to each other, and purified water for dissolving these components.